CD22 is a 135-kD type I transmembrane sialoglycoprotein of the immunoglobulin (Ig) superfamily. CD22 expression is specific to B cells can be detected in the cytoplasm of pro-B and pre-B cells, as well as on the surface of IgM+ B cells upon the appearance of IgD, but not on terminally-differentiated plasma cells (Tedder et al., 1997, Ann Rev Immunol 15:481-504). CD22 is strongly expressed on follicular, mantle and marginal-zone B cells, but is weakly present in germinal B cells (Dorner & Goldenberg, 2007, Ther Clin Risk Manag 3:954-59). CD22 is an inhibitory co-receptor that downmodulates B-cell receptor (BCR) signaling by setting a signaling threshold that prevents overstimulation of B cells (Nitschke, 2005, Curr Opin Immunol 17:290-97).
Structurally, CD22 in its extracellular region comprises 7 immunoglobulin-like domains, of which the two N-terminal domains are involved in ligand binding (Law et al., 1995, J Immunol 155:3368-76). The cytoplasmic tail of CD22 contains 6 conserved tyrosine (Y) residues, four of which (Y762, Y796, Y822, and Y842) in hCD22 are considered to be localized within the immunoreceptor tyrosine-based inhibition motifs (ITIM) (Wilson et al., 1991, J Exp Med 173:137-46; Ravetch & Lanier, 2000, Science 290:84-9; Sato et al., 1998, Semin Immunol 10:287-97). These tyrosine residues also fit in the specific internalization motifs of YXXØ (where Ø denotes a hydrophobic residue) (Bonifacino et al., 2003, Ann Rev Biochem 72:395-447), which mediate the recruitment of CD22 to clathrin-coated pits and are required for the internalization of CD22 (John et al., 2003, J Immunol 170″3534-43). Whereas endocytosed CD22 has been documented to be degraded intracellularly (Shan & Press, 1995, J Immunol 154:4466-75), a recent report contended that CD22 instead is constitutively recycled back to the cell surface (O'Reilly et al., 2011, J Immunol 186:1554-63). Functionally, CD22 recognizes α2,6-linked sialic acids on glycoconjugates in both cis (on the same cell) and trans (on different cells) interactions, and modulates B cells via interaction with CD79a and CD79b, the signaling components of the BCR complex (Razi & Varki, 1998 95:7469-74; Engels et al., 1993, J Immunol 150:4719-32; Leprince et al., 1993, Proc Natl Acad Sci USA 90:3236-40). Crosslinking the BCR with cognate antigens or appropriate antibodies against membrane immunoglobulin (mIg) on the cell surface induces translocation of the aggregated BCR complex to lipid rafts (Petri et al., 2000, J Immunol 165:1220-27), where CD79a, CD79b and CD22, among others, are phosphorylated by Lyn (Smith et al., 1998, J Exp Med 187:807-11), which in turn triggers various downstream signaling pathways, culminating in proliferation, survival, or death (Niiro et al., 2002, Nat Rev Immunol 2:945-56).
Antibodies against CD22, such as epratuzumab (hLL2), have been used for treatment of a variety of cancers and autoimmune diseases, including but not limited to acute lymphoblastic leukemia (Hoelzer et al., 2013, Curr Opin Oncol 25:701-6), chronic lymphocytic leukemia (Macromatis & Cheson, 2004, Blood Rev 18:137-48), non-Hodgkin's lymphoma (Leonard et al., 2004, Clin Cancer Res 10:5327-34; Dorner & Goldenberg, 2007), follicular lymphoma (Illidge & Morchhauser, 2011, Best Pract Res Clin Haematol 24:279-93), diffuse large B-cell lymphoma (Micallef et al., 2011, Blood 118:4053-61), mantle cell lymphoma (Sharkey et al., 2012, Mol Cancer Ther 11:224-34), systemic lupus erythematosus (Dorner & Goldenberg, 2007; Strand et al., 2014, Rheumatology 53:502-11; Wallace & Goldenberg, 2013, Lupus 22:400-5; Wallace et al., 2013, Rheumatology 52:1313-22; Wallace et al., 2014, Ann Rheum Dis 73:183-90), and primary Sjögren's syndrome (Steinfeld et al., 2006, Arthritis Res Ther 8:R129; Dorner & Goldenberg, 2007). Because CD22 regulates B-cell functions and survival, it is an important link for modulating humoral immunity and proliferation of B-cell lymphomas and a target for therapeutic antibodies in cancer and autoimmune disease (Dorner & Goldenberg, 2007).
Epratuzumab (hLL2), a humanized monoclonal antibody specific for human CD22 (Leung et al., 1995, Mol Immunol 32:1413-27), is currently under clinical investigation for the treatment of non-Hodgkin lymphoma (NHL) (Leonard et al., 2004, Clin Cancer Res 10:5327-34; Leonard et al., 2009, Cancer 113:2714-23), pediatric and adult acute lymphoblastic leukemia (Raetz et al., 2008, J Clin Oncol 26:3756-62; Advani et al., 2014, Br J Haematol 165:504-9), systemic lupus erythematosus (SLE) (Wallace et al., 2013, Lupus 22:400-5; Wallace et al., 2013, Rheumatology 52:1313-22; Strand et al., Rheumatology 53:502-11), and has shown promise in patients with primary Sjögren's syndrome in a phase I/II study (Steinfeld et al., 2006, Arthritis Res Ther 8:R129). Research on anti-CD22 antibodies, which can be either blocking or nonblocking (Tuscano et al., 1996, Blood 87:4723-30), has also led to intriguing observations that CD22 may positively or negatively affect BCR-mediated signaling pathways, with the ultimate outcome depending on the characteristics of the B cells (differentiation stage, expression of BCR isotype, and being malignant, abnormal, or normal) (Tuscano et al., 1996, Blood 87:4723-30; Pezzutto et al, 1987, J Immunol 138:98-103; Pezzutto et al., 1988, J Immunol 140:1791-95; Nitschke et al., 2005, Curr Opin Immunol 17:290-97). Thus, fully understanding the role of CD22 in B-cell malignancies, as well as B-cell-implicated autoimmune diseases, is of considerable importance for improving CD22-targeted therapies.
As a single agent, epratuzumab is well-tolerated, depleting circulating B cells transiently in NHL patients (Leonard et al., 2009, Cancer 113:2714-23), and by an average of 35% at 18 weeks in SLE patients (Dorner et al., 2006, Arthritis Res Ther 8:R74). When evaluated in vitro, epratuzumab displayed modest antibody-dependent cellular cytotoxicity, but no complement-dependent cytotoxicity (Carnahan et al., 2007, Mol Immunol 44:1331-41), and was shown to inhibit the proliferation of B cells from patients with SLE, but not from normal donors, under all culture conditions (Jacobi et al., 2008, Ann Rheum Dis 67:450-57). Additional studies with B cells from SLE patients indicated that (i) binding of epratuzumab was particularly enhanced on CD27− compared to CD27+ B cells (Daridon et al., 2010, Arthritis Res Ther 12:R204); (ii) a decrease of CD62L and β7 integrin and an increase of β1 integrin in the cell surface expression, as well as an enhanced migration towards CXCL12, were noted for CD27− B cells preferentially (Daridon et al., 2010); and (iii) the in vivo effects of epratuzumab in SLE patients included an immediate and sustained (up to 18 weeks) decrease in CD22 surface expression on circulating CD27− and CD27+ B cells (Daridon et al., 2010). More recently, pre-incubation with F(ab′)2 of epratuzumab was reported to inhibit calcium mobilization and phosphorylation of Syk and PLCγ2 in normal human B cells after BCR stimulation (Sieger et al., 2013, Arthritis Rheum 65:770-79), and the ability of epratuzumab-based agents to effectively mediate Fc-dependent trogocytosis of multiple B-cell surface markers by FcγR-bearing cells, was established (Rossi et al., 2013, Blood 122:3020-29; Rossi et al., 2014, PLoS ONE 9:e98315).
In cell lines and xenografts of human Burkitt lymphoma, soluble epratuzumab, although capable of phosphorylating CD22 (Carnahan et al., 2003, Clin Cancer Res 9:3982s-90s) and translocating CD22 to lipid rafts (Qu et al, 2008, Blood 111:2211-19), as demonstrated in vitro, was not cytotoxic or cytostatic (Carnahan et al., 2007, Mol Immunol 44:1331-41; Qu et al., 2008), and displayed only minimal toxicity even when crosslinked by goat anti-human IgG Fcγ (GAH) (Carnahan et al., 2007). On the other hand, in vitro cytotoxicity of epratuzumab comparable to that achievable with anti-IgM (10 μg/mL) could be consistently demonstrated in Ramos and D1-1, a subclone of Daudi selected for a high expression of membrane IgM (mIgM), when the antibody was immobilized to plastic plates, or added in combination with suboptimal amounts of anti-IgM (for example, 1 μg/mL or less) along with GAH (Carnahan et al., 2007). Despite all of this knowledge, how epratuzumab kills or modulates normal and malignant B cells in patients, and inhibits the growth of lymphoma lines in vitro upon immobilization, remains poorly understood. A need exists in the field for improved methods of use of anti-CD22 antibodies, based on their mechanism(s) of action of anti-cancer and/or anti-autoimmune disease activity.